Lewis & Short

†† burdo, ōnis (collat. form burdus, i, Acron. ad Hor. C. 3, 27, 7), m., = [??], a mule (the offspring of a horse and she-ass, while mulus is the offspring of an ass and a mare; v. Isid. Orig. 12, 1, 61), esp. used for carrying litters, Dig. 32, 49: onus duorum burdonum, Vulg. 4 Reg. 5, 17.